One thing that comes to mind is that some of the interest he inherited from his aunt may already be included in wells that are producing. The Texas unclaimed property site might be logical to check for royalties the State might be holding before doing anything further. https://claimittexas.org/ Unless specifically excluded in the deed covering a sale of mineral interest, ownership of any previously accrued royalties would also pass to the buyer.
Next I’d make sure I knew the extent of interest I owned. Was his aunt’s estate probated and did your husband receive a deed transferring the interest? Post what you know here (names, survey or abstracts, deed references) and I’m sure folks will guide you on logical next steps to take.
You mentioned the offer coming from an individual. You might ask them to email you their understanding of the make up of the mineral interest they are offering to buy. They don’t have to tell you but if they won’t share anything I’d be suspicious…obviously they know some details.
Also, I wouldn’t let anyone rush me on making a decision although that is often the approach used. With the general state of the oil business this isn’t the best time to be selling minerals, and the number of purchase offers being made these days has definitely decreased from pre-Covid times, but there is no reason to commit in any way until you are confident you understand what is involved.